1. Field of the Invention
The present invention relates to stimulable phosphors, methods of making same, and X-ray image transforming sheets. More specifically, the present invention relates to stimulable phosphors activated by divalent europium, and the uses thereof. Such phosphors emit a light having a peak near to 400 nm when excited by radiation such as X-rays, electron beams or ultraviolet rays, and further, release a light having a peak near to 400 nm when later irradiated with visible or infrared light. The latter released light is called photostimulated luminescence, and is used for storing the energy of radiation and transforming it into visible light. In particular, such released light is used for transforming radiation images into visible images and one of the most important applications thereof relates to X-ray image production for medical use.
2. Description of the Related Art
In X-ray image production using stimulable phosphors, X-rays transmitted through a part of a living body, such as a breast, are irradiated to a stimulable phosphor sheet or panel containing a stimulable phosphor, and the stimulable phosphor sheet or panel is then scanned by a laser beam. When excited by laser beams, stimulable phosphors release energy stored in crystals of the phosphor as stimulated luminescence of near-ultraviolet light corresponding to the stored energy. The near-ultraviolet light is detected by a detector such as a photoelectron multificator and then transformed into an electric signal. The electric signal is treated and displayed on a cathode ray tube (CRT), or is used to modulate the intensity of another laser beam scanning and exposing a silver salt film, which is then developed to produce a visible image.
This method of transforming a laser beam into a visible image has remarkable useful features, such as the following.
(1) It is possible to reduce the amount of X-ray exposure.
(2) A satisfactory image can be produced by persons not skilled in the art, since stimulable phosphors have an X-ray sensitive range wider than that of a silver salt film, and thus the adjustment of the X-ray exposure for producing an image is easy.
(3) Since the image is transformable into an electric signal, various image treatments such as contour emphasizing can be easily effected.
(4) After producing an image, the X-ray image transforming sheet can be used repeatedly by irradiating the phosphor sheets with a stimulating light to release all stored energy and return the sheets to their original state.
Some stimulable phosphors used for these purposes, for example, cerium and samarium-activated strontium sulfide phosphors (SrS:Ce,Sm), europium and samarium-activated lanthanum oxisulfide phosphors (La.sub.2 O.sub.2 S:Eu,Sm), and manganese and halogen-activated zinc cadmium sulfide phosphors [(ZnCd)S:Mn,X; where X is a halogen], as disclosed in G.B. Patent No. 1462769 or U.S. Pat. No. 3,859,527, are known. Japanese Unexamined Patent Application (Kokai) No. 55-12143 discloses stimulable phosphors having the formula (Ba.sub.1-x-y Mg.sub.x Ca.sub.y) FX:Eu where X is Br or Cl; Japanese Unexamined Patent Publication (Kokai) No. 55-84389 discloses stimulable phosphors having the formula BaFX:Ce,A where X is Cl, Br or I and A is In, Tl, Gd, Sm or Zr; and Japanese Unexamined Patent Publication (Kokai) No. 60-84381 discloses stimulable phosphors having the formula MX.sub.2 aMX'.sub.2 :Eu where M is Ba, Sr or Ca and X and X' are Cl, Br or I.
Nevertheless, these stimulable phosphors are not adequate for intended industrial applications.
Namely, the stimulable phosphors disclosed in G.B. Patent No. 1,462,769 and U.S. Pat. No. 3,859,527 have low X-ray sensitivity.
The stimulable phosphors disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 55-12143 and 55-84389 have practical reading sensitivities when subjected to stimulation by visible lasers, e.g., helium neon lasers, but can not be stimulated by infrared semiconductor lasers made of a material such as GaAs, GaAlAs or InPGaAs, and therefore, are not practical. The stimulable phosphors disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 60-84381 and 55-12143 have spectrums of stimulated luminescence for reading that extend to slightly longer wave lengths, but do not have sufficient characteristics for practical reading by semiconductor lasers.
The stimulable phosphors disclosed in Japanese Unexamined Patent Publication (Kokai) No. 60-84381 are MFXX':Eu. These phosphors are similar to the MFX:Eu phosphors disclosed in Japanese Unexamined Patent Publication (Kokai) No. 55-12143 except that a portion of the F is replaced by a halogen X' which is different from F and X, but do not have a practical sensitivity for reading by a semiconductor laser. When the intensity of stimulated luminescence measured by stimulating the phosphors MFXX':Eu by light having a wave length of 780 nm is compared with that of MFX:Eu, it is shown that MFX:Eu exhibits a luminescence intensity in a near-infrared wave length range that is larger than the almost zero luminescence of the other stimulable phosphors. However, the luminescence intensity of MFX:Eu is still too low for practical use.
The object of the present invention is to provide novel stimulable phosphors capable of releasing luminescence having a practical intensity when stimulated by a near-infrared ray of a semiconductor laser. The semiconductor lasers of the present invention comprise those having a high output at a wave length longer than 500 nm, particularly longer than 760-780 nm. Although semiconductor lasers having an output at higher wave lengths, i.e., visible light, have been developed, semiconductor lasers having a sufficient output for X-ray image transformation are limited to those having the above wave lengths, More specifically, an output power of at least 20 mW is necessary, which limits the semiconductor laser to the above lasers.